Satellite images of Kranji woodland show over 8ha of forested land cleared since last March.
This is almost half of the site slated for the future Agri-Food Innovation Park - the park is the size of 33 football fields - and more than 10 per cent of the 70ha woodland.
Developer JTC Corporation told The Straits Times on Tuesday that the "erroneous" clearing started last December, before environmental assessments were complete.
When contacted, JTC told ST it is still investigating.
Green patches totalling at least 8ha - or 11 football fields - were cleared by Jan 25 this year, according to satellite Sentinel-2B, used as part of the European Union's Copernicus earth-monitoring project.
Data on Global Forest Watch - a monitoring service run by the World Resources Institute that uses satellite data - also revealed forested land disappearing from March last year.
In addition, photos on Google Street View showed a sign saying "proposed site clearance and earth works at Kranji Road for Plot 9" at the deforested land next to Kranji Road last September.
On Tuesday, a JTC spokesman said the mistake was discovered on Jan 13, over a month before aerial images of the deforestation were posted on social media last Sunday.
JTC added that an environmental specialist was engaged to carry out a biodiversity baseline study last December to work out an environmental monitoring and management plan for specified plots of land within the area. These were expected to be completed in April.
Following these assessments, the statutory board said, it had "plans to engage key stakeholders, including nature groups, to discuss its development plans".
On discovering the error on Jan 13, JTC ordered all clearing works to stop and issued a stern warning to the contractor, it added.
According to a book published by Nature Society (Singapore) titled The Green Rail Corridor, the forested area cleared for the park is part of Kranji Woodland-Scrubland, spanning about 70ha.
A survey of the rail corridor found 47 species of birds - a mix of resident and migratory species - which account for 12 per cent of the total number of species recorded in Singapore.
On Tuesday, the National Parks Board said it was investigating the unauthorised clearance.
Under the Parks and Trees Act, it is illegal to fell a tree with a girth exceeding 1m growing on any vacant land, whether within or outside a tree conservation area, except with the board's approval.